fix: renderAsync honors monkey-patched renderer.render

[ShenQingchuan]

Fixes #33

Problem

renderAsync() calls renderer.renderAsync() directly, while render() goes through renderer.render(). Plugins that monkey-patch md.renderer.render — a widespread markdown-it ecosystem pattern, e.g. @mdit-vue/plugin-headers (used by VitePress to extract page headers into env) — are silently bypassed under renderAsync(), and their env writes never happen.

Fix

Renderer.renderAsync now detects whether render has been overridden or monkey-patched on the instance (this.render !== Renderer.prototype.render) and delegates to it, so patched logic applies on the async path too.

Trade-off, made explicit: on the delegated sync path, async rules still throw the descriptive async rule detected error — same behavior as render(). Plugins that patch render are synchronous collectors by nature, so this combination is rare; when it does happen, the error message points the way.

Tests

Three new tests in tests/renderer.test.ts:

• renderAsync honors a monkey-patched render (env write lands, output correct)
• the parallel path still runs when render is untouched
• a patched render combined with an async rule still surfaces the async-rule error

Full suite: 963 passed (960 baseline + 3 new), lint clean.

[codspeed]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing ShenQingchuan:fix/render-async-honor-render-patch (2c907c7) with main (1c1c7cb)}